1. Field of the Invention
The present invention relates generally to graphic input systems and more particularly to digitizing tablets utilized in connection with graphic input systems.
2. Brief Description of the Prior Art
Graphic input systems are conventionally known which operate to generate electrical signals that represent the position of a pen or other mechanism relative to an input surface. The electrical signals generated are then stored so that the information written on the input surface may be reproduced at some later point.
Examples of a graphic input system are illustrated in U.S. Pat. Nos. 4,570,033 and 4,552,991, each to Leonard R. Hulls, our co-pending U.S. patent application Ser. No. 08/548,283 (hereinafter the "'283 application") filed Oct. 25, 1995, which are each assigned to the assignee of the present invention, and U.S. Pat. Nos. 3,801,733 and 3,647,963 to Bailey, all of which are incorporated by reference herein. As illustrated in the '283 application, a graphic input system incorporates as the input surface a digitizing tablet which generally is comprised of a flat surface and a system of grid wires usually mounted just beneath or contained within the flat surface. In operation of the graphic input system, graphical material to be digitized, such as a document, is placed upon the surface of the digitizing tablet and a specialized pen is positioned as required upon the document to digitize the information relative to an XY coordinate measurement system.
Most of the conventional methods used for implementing an XY coordinate system of a digitizer tablet, such as is shown in the foregoing patents, employ a grid of conductors of some type mounted under the work surface and a coil, which is driven with AC current, mounted in the digitizer pointing device. The coil generates an alternating magnetic field which couples a voltage into the grid of conductors. The voltage in the grid is sensed and processed to determine the XY position of the magnetic field. It is also possible to reverse the roles of the grid and coil, so that the grid is driven and the coil is the sense element.
Many of the most effective methods utilized for implementing the needed measurement system, especially for large scale digitizers, involves using a serpentine shape for the basic grid elements. Grid elements of this type are placed at right angles to each other to generate both X and Y coordinate values. As illustrated in the foregoing prior art, layers of grid elements are combined to produce an effective XY coordinate measurement system. All of these methods rely on the fact that the amplitude of the voltage, induced in the grid element by the AC excited coil, varies in an approximately sinusoidal fashion as the coil is moved across the coil elements perpendicular to the main conductor paths. When the grids are properly constructed, as described in the above referenced patents, this voltage pattern is largely independent of the position of the coil along the axis parallel to the main conductor paths. There are edge effects which cause the measurement system to break down as the coil approaches the edges of the grid pattern. For most conventional digitizing tablet applications, an inactive area a few centimeters wide around the outside of the work surface is perfectly acceptable.
Digitizing tablets have been in use for years and application of the technology has diversified dramatically over the past few years. For example, digitizing tablets are no longer restricted to use as CAD station accessories and are finding application in a variety of other office product applications, such as in electronic whiteboards and interactive computer display systems.
Whiteboards are ordinarily utilized for taking notes or presenting and sharing ideas in small group meetings and in more formal presentations. Electronic whiteboards were developed to provide a means of capturing, electronically, the notes and diagrams that are written on the whiteboard during a meeting, which allows the information that was written on the board to be saved and distributed after the meeting is over. A limitation observed with electronic whiteboards is that they are relatively small in size and, as a result, have more limited applications than conventional whiteboards which are often used in classrooms and meeting rooms that are very large, with the whiteboard sometimes occupying an entire wall.
A similar requirement exists in the digitizer technology when it is used in interactive computer display systems, such as for computer generated presentations. As computers and screen projector systems have become smaller and more portable, there is seen a demand for the digitizer, which serves as an interactive projection surface, to also be made portable.
In view of the prior art noted above, there is seen a need for a digitizing tablet that is larger in size and, where desired, can be made portable for ease of use.